1. Field of the Invention
The present invention relates to a common-rail fuel injection device for an internal combustion engine.
2. Description of the Related Art
A common-rail fuel injection device is known as a fuel injection device for an internal combustion engine (hereinafter also referred to simply as “engine”). In the common rail system, fuel pressurized by a fuel supply pump (supply pump) is accumulated in a common rail. The pressurized fuel is supplied from the common rail to fuel injection valves through fuel supply pipes (fuel injection pipes).
Thus, at the time of fuel injection from the fuel injection valve, the fuel injection pressure of the fuel to be injected into each cylinder (combustion chamber) is increased so that the particle diameter of the fuel injected into the cylinder is reduced. As a result, the vaporization or atomization rate of the fuel injected into the combustion chamber is increased so that complete combustion is promoted, thereby reducing unburned substances contained in exhaust gas (such as hydrocarbon and carbon monoxide).
In addition, the volume of the fuel accumulated in the common rail under the pressurized state is relatively large, and hence the decrease amount of the “pressure of the fuel supplied to the fuel injection valve (injection-valve fuel pressure)” is small immediately after the end of fuel injection. Therefore, the fuel injection can be repeated within a short period of time, thereby being capable of achieving multi-stage injection for injecting the fuel into a single cylinder a plurality of times in one cycle (sequential fuel injection involving pre-injection, main injection, after-injection, and post-injection).
Even in the common-rail fuel injection device, however, the injection-valve fuel pressure is temporarily decreased to some extent when the fuel is injected. As a result, when further fuel injection is performed immediately after the end of fuel injection (for example, when second pre-injection is performed after first pre-injection), the actual fuel injection amount may become smaller than the expected fuel injection amount. In addition, the particle diameter of the injected fuel may become larger.
In view of the above, one of the related-art common-rail fuel injection devices (hereinafter also referred to as “related-art device”) is applied to an engine including four cylinders, and includes four fuel injection valves each having two fuel supply ports and being arranged for a corresponding one of the cylinders.
In this related-art device, one of the two fuel supply ports of the first fuel injection valve is directly connected to a common rail through a fuel supply pipe, whereas the other one of the fuel supply ports of the first fuel injection valve is directly connected to one of the two fuel supply ports of the second fuel injection valve through an injection-valve connection pipe. The other one of the fuel supply ports of the second fuel injection valve is directly connected to one of the two fuel supply ports of the third fuel injection valve through an injection-valve connection pipe. The other one of the fuel supply ports of the third fuel injection valve is directly connected to one of the two fuel supply ports of the fourth fuel injection valve through an injection-valve connection pipe. The other one of the fuel supply ports of the fourth fuel injection valve is directly connected to the common rail through a fuel supply pipe. Thus, each fuel injection valve injects the fuel supplied through the two fuel supply ports into the cylinder (see, for example, International Patent WO2011/085858A).
According to the related-art device, the pressurized fuel is supplied to each fuel injection valve through the two fuel supply ports of each fuel injection valve, thereby being capable of reducing the magnitude of the decrease amount of the fuel injection pressure (injection-valve fuel pressure) immediately after the end of fuel injection as compared to a case where the fuel injection valve has one fuel supply port alone.
Even in the related-art device, however, the injection-valve fuel pressure is decreased to some extent immediately after the end of fuel injection, and is increased afterwards. As a result, the injection-valve fuel pressure fluctuates over time. The fluctuation of the injection-valve fuel pressure (hereinafter also referred to simply as “fuel pressure fluctuation”) propagates to another fuel injection valve through the fuel in the injection-valve connection pipe. As a result, the amount of fuel injected in actuality may become significantly different from the expected amount of fuel, or the particle diameter of the injected fuel may become larger.
Now, the above-mentioned fuel pressure fluctuation and its influence are further described with reference to an example of the related-art device having a schematic configuration illustrated in FIG. 9.
The related-art device of FIG. 9 includes a common rail 91 and a first fuel injection valve 92a to a fourth fuel injection valve 92d. The common rail 91 and the first fuel injection valve 92a are connected by a first fuel supply pipe 93a. The common rail 91 and the fourth fuel injection valve 92d are connected by a second fuel supply pipe 93b. 
The first fuel injection valve 92a and the second fuel injection valve 92b are connected by a first injection-valve connection pipe 94e. The second fuel injection valve 92b and the third fuel injection valve 92c are connected by a second injection-valve connection pipe 94b. The third fuel injection valve 92c and the fourth fuel injection valve 92d are connected by a third injection-valve connection pipe 94c. 
FIG. 4 is a graph for showing results of measurement conducted by the inventors of the present invention on “how the fuel pressure fluctuation caused by the fuel injection from the second fuel injection valve 92b propagates to the first fuel injection valve 92a” when the fuel injection is performed in an order of the first fuel injection valve 92a, the third fuel injection valve 92c, the fourth fuel injection valve 92d, and the second fuel injection valve 92b. 
The solid line Lp1 of FIG. 4 indicates a change of the injection-valve fuel pressure of the second fuel injection valve 92b at the time of performing fuel injection from the second fuel injection valve 92b. The solid line Lp2 of FIG. 4 indicates a change of the injection-valve fuel pressure of the first fuel injection valve 92a at that time. As understood from the ellipse Ce1 to the ellipse Ce4 of FIG. 4, the injection-valve fuel pressure of the first fuel injection valve 92a fluctuates along with the fuel injection from the second fuel injection valve 92b. 
Therefore, for example, when the timing of post-injection from the second fuel injection valve 92b and the timing of main injection from the first fuel injection valve 92a are close to each other (see FIG. 2), the amount of fuel injected from the first fuel injection valve 92a in actuality may become significantly different from the expected amount of fuel. Further, when the fuel is injected from the first fuel injection valve 92a at a low injection-valve fuel pressure of the first fuel injection valve 92a (that is, at a trough of the fuel pressure fluctuation), the particle diameter of the injected fuel may become larger.